As a valve train for an internal combustion engine in which the valve operating properties of an engine valve are changed in accordance with an oscillating position of an oscillating member which supports a transmission mechanism which transmits the valve drive force of a valve operating cam to the engine valve, there is a valve train disclosed, for example, in Japanese Patent Unexamined Publication No. JP-A-7-91217. The valve train disclosed in the JP-A-7-91217 includes a drive shaft which is driven to rotate by an internal combustion engine, a camshaft which is disposed rotatably on an outer circumference of the drive shaft and which is provided rotatably on a cylinder head, a cam formed on the camshaft, a disk housing adapted to oscillate in a radial direction relative to the drive shaft about a pivot pin acting as a fulcrum, an annular disk which is rotatably supported on an inner circumferential surface of the disk housing, a drive mechanism for oscillating the disk housing and a rocker arm which is pivot supported on a rocker shaft which is supported, in turn, on the disk housing at one end portion thereof and which abuts with the cam and an inlet valves.
Then, when the disk housing is caused to oscillate by the drive mechanism, the center of the annular disk becomes eccentric to the axial center of the drive shaft, so that the rotational phase difference and rotational angular velocity ratio of the cam and the drive shaft change, whereby the operation angle of the inlet valve is changed. At the same time, the pivot fulcrum position of the rocker arm is changed through the displacement of the rocker shaft which oscillates together with the disk housing, and the other end portion of the rocker arm shits over an upper surface of a valve lifter along a diametrical direction thereof, whereby a rocker ratio relative to the inlet valve is changed so that the valve lift amount is changed.
In the related art disclosed in the JP-A-7-91217, since a cam abutment position between the cam and the rocker arm is situated substantially on a straight line which passes a rotational center line of the cam and an oscillating center line of the disk housing on a plane which intersects with the oscillating center line at right angles, a moment acting on the disk housing based on a valve drive force acting at the cam abutment position is reduced when the rocker arm is in abutment with a lobe portion of the cam, whereby the drive force of the drive mechanism which is necessary to oscillate the disk housing is reduced. However, since the rocker arm abuts with both the cam and the inlet valve, the related art has the following drawbacks.
Namely, when attempting to maintain the closed state of the inlet valve with the rocker arm being allowed to abut with a base circle of the cam, since the cam abutment position cannot be shifted largely on the base circle of the cam, the oscillating amount of the pivot support position of the rocker arm by virtue of oscillation of the disk housing is limited to a relatively small value, and this disables a large change in rotational phase difference, rotational angular velocity ratio and rocker ratio, thereby making it difficult to increase the control range of the opening and closing timings and maximum lift amount of the inlet valve. In addition, since the pivot fulcrum position and pivot pin position are determined unconditionally from the positional relation between the cam and the inlet valve, the degree in freedom in arrangement of the rocker arm and the pivot pin is limited, and with, for example, an internal combustion engine having a relatively compact cylinder head, the interference of the valve train with peripheral members of the engine which are disposed therearound cannot be avoided, and it becomes difficult to dispose the valve train of the related art within the limited space, resulting in the possible occurrence of a risk that the related art valve train cannot be adopted for the aforesaid internal combustion engine. In addition, when attempting to have a specific positional relation between the abutment position of the rocker arm with the inlet valve and the oscillating center line, in addition to the positional relation among the cam abutment position, the rotational center line and the oscillating center line, the degree of freedom is reduced further.